Method of fabricating a high-voltage transistor with an extended drain structure

ABSTRACT

A method for fabricating a high-voltage transistor with an extended drain region includes forming in a semiconductor substrate of a first conductivity type, first and second trenches that define a mesa having respective first and second sidewalls; then partially filling each of the trenches with a dielectric material that covers the first and second sidewalls. The remaining portions of the trenches are then filled with a conductive material to form first and second field plates. Source and body regions are formed in an upper portion of the mesa, with the body region separating the source from a lower portion of the mesa. It is emphasized that this abstract is provided to comply with the rules requiring an abstract that will allow a searcher or other reader to quickly ascertain the subject matter of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. 37 CFR 1.72(b).

RELATED APPLICATIONS

This application is a continuation of application Ser. No. 11/807,531filed May 29, 2007 now U.S. Pat. No. 7,745,291, which is a continuationof application Ser. No. 11/021,933 filed Dec. 23, 2004, now U.S. Pat.No. 7,253,042, which is a continuation of application Ser. No.10/722,792 filed Nov. 25, 2003, now U.S. Pat. No. 6,838,346, which is acontinuation of application Ser. No. 10/278,551 filed Oct. 22, 2002, nowU.S. Pat. No. 6,750,105, which is a division of application Ser. No.09/948,879 filed Sep. 7, 2001, now U.S. Pat. No. 6,635,544, and is alsorelated to application Ser. No. 10/278,432 filed Oct. 22, 2002, now U.S.Pat. No. 6,667,213, which applications are assigned to the assignee ofthe present application.

FIELD OF THE INVENTION

The present invention relates to semiconductor devices fabricated in asilicon substrate. More specifically, the present invention relates tofield-effect semiconductor transistor structures capable of withstandinghigh voltages.

BACKGROUND OF THE INVENTION

High-voltage, field-effect transistors (HVFETs) are well known in thesemiconductor arts. Most often, HVFETs comprise a device structure thatincludes an extended drain region that supports the applied high-voltagewhen the device is in the “off” state. HVFETs of this type are commonlyused in power conversion applications such as AC/DC converters foroffline power supplies, motor controls, and so on. These devices can beswitched at high voltages and achieve a high blocking voltage in the offstate while minimizing the resistance to current flow in the “on” state.The blocking or breakdown voltage is generally denoted as Vbd. Theacronym Rsp refers to the product of the resistance and surface area,and is generally used to describe the on-state performance of thedevice. An example of a prior art HVFET having an extended drain regionwith a top layer of a conductivity type opposite that of the extendeddrain region is found in U.S. Pat. No. 4,811,075.

In a conventional HVFET the extended drain region is usually lightlydoped to support high voltages applied to the drain when the device isoff. The length of the extended drain region is also increased to spreadthe electric field over a larger area so the device can sustain highervoltages. However, when the device is on (i.e., conducting) currentflows through the extended drain region. The combined decrease in dopingand increase in length of the extended drain region therefore have thedeleterious effect on the on-state performance of the device, as bothcause an increase in on-state resistance. In other words, conventionalhigh-voltage FET designs are characterized by a trade-off between Vbdand Rsp.

To provide a quantitative example, a typical prior art vertical HVFET(NMOS-type) may have a Vbd of 600V with a Rsp of about 16 ohm-mm².Increasing the length of the extended drain would affect deviceperformance by increasing Vbd beyond 600V at the expense of a higher Rspvalue. Conversely, reducing the length of the extended drain wouldimprove the on-state resistance to a value below 16 ohm-mm², but such achange in the device structure would also cause Vbd to be reduced toless than 600V.

A device structure for supporting higher Vbd voltages with a low Rspvalue is disclosed in U.S. Pat. Nos. 4,754,310, 5,438,215, and also inthe article entitled, “Theory of Semiconductor Superjunction Devices” byT. Fujihira, Jpn. J. Appl. Phys., Vol. 36, pp. 6254-6262, October 1977.In this device structure the extended drain region comprises alternatinglayers of semiconductor material having opposite conductivity types,e.g., PNPNP . . . . As high voltage is applied to the layers of oneconductivity type, all of the layers are mutually depleted of chargecarriers. This permits a high Vbd at much higher conducting layer dopingconcentrations as compared to single layer devices. The higher dopingconcentrations, of course, advantageously lower the Rsp of thetransistor device. For example, in the article entitled, “A newgeneration of high voltage MOSFETs breaks the limit line of silicon” byG. Deboy et al., IEDM tech. Digest, pp. 683-685, 1998, the authorsreport a vertical NMOS device with a Vbd of 600V and a Rsp of about 4ohm-mm².

Another approach to the problem of achieving high-voltage capability isdisclosed in the paper, “Realization of High Breakdown Voltage in ThinSOI Devices” by S. Merchant et al., Proc. Intl. Symp. on Power Devicesand ICs, pp. 31-35, 1991. This paper teaches an extended drain regionthat comprises a thin layer of silicon situated on top of a buried oxidelayer disposed on top of a semiconductor substrate. In operation, theunderlying silicon substrate depletes charge from the thin silicon layerat high voltages. The authors claim that high values of Vbd are obtainedas long as the top silicon layer is sufficiently thin and the buriedoxide layer is sufficiently thick. For instance, a lateral NMOS devicewith Vbd of 600V and Rsp of about 8 ohm-mm² is obtained using thisapproach.

Other background references of possible interest to those skilled in theart include U.S. Pat. Nos. 6,184,555, 6,191,447, 6,075,259, 5,998,833,5,637,898, International Application No. PCT/IB98/02060 (InternationalPublication No. WO 99/34449), and the article, “High Performance 600VSmart Power Technology Based on Thin Layer Silicon-on-Insulator” by T.Letavic et al., Proc. ISPSD, pp. 49-52, 1997.

Although the device structures described above achieve high Vbd withrelatively low on-state resistance as compared to earlier designs, thereis still an unsatisfied need for a high-voltage transistor structurethat can support still higher voltages while achieving a much loweron-state resistance.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is illustrated by way of example, and notlimitation, in the figures of the accompanying drawings, wherein:

FIG. 1 is a cross-sectional side view of a vertical high-voltage,field-effect transistor (HVFET) device structure in accordance with oneembodiment of the present invention.

FIG. 2 is a cross-sectional side view of one embodiment of a lateralHVFET fabricated in accordance with the present invention.

FIG. 3A is a top view of lateral HVFET fabricated in accordance withanother embodiment of the present invention.

FIG. 3B is a cross-sectional side view of the lateral HVFET shown inFIG. 3A, taken along cut lines A-A′.

FIG. 4 is a cross-sectional side view of another embodiment of avertical HVFET device structure fabricated according to the presentinvention.

FIGS. 5A-5K are cross-sectional side views of a vertical HVFET devicestructure taken at various stages in a fabrication process in accordancewith yet another embodiment of the present invention.

FIG. 6 is a cross-sectional side view of still another embodiment of avertical HVFET device structure fabricated according to the presentinvention.

DETAILED DESCRIPTION

A high-voltage field-effect transistor having an extended drain regionand a method for making the same is described. The HVFET has a lowspecific on-state resistance and supports high voltage in the off-state.In the following description, numerous specific details are set forth,such as material types, doping levels, structural features, processingsteps, etc., in order to provide a thorough understanding of the presentinvention. Practitioners having ordinary skill in the semiconductor artswill understand that the invention may be practiced without many ofthese details. In other instances, well-known elements, techniques, andprocessing steps have not been described in detail to avoid obscuringthe invention.

FIG. 1 is a cross-sectional side view of a vertical n-channel (i.e.,NMOS) HVFET 20 in accordance with one embodiment of the presentinvention. It should be understood that the elements in the figures arerepresentational, and are not drawn to scale in the interest of clarity.It is also appreciated that a p-channel transistor may be realized byutilizing the opposite conductivity types for all of the illustrateddiffusion/doped regions. Furthermore, although the figure appears toshow two separate devices, those of skill will understand that suchtransistor structures are commonly fabricated in an annular,inter-digitated, or otherwise replicated manner.

The device structure of FIG. 1 includes an insulated-gate, field-effecttransistor (IGFET) having a gate 30 (comprised, for example, ofpolysilicon), and a gate-insulating layer 29 that insulates gate 30 fromthe underlying semiconductor regions. Gate-insulating layer 29 maycomprise ordinary silicon dioxide or another appropriate dielectricinsulating material. The extended drain region of vertical HVFET 20comprises two or more parallel N-type drift regions 22 situated betweenp-type body regions 26 and extending down to the N+ substrate 21. Forinstance, FIG. 1 shows drift region 22 a extending from beneath gateoxide 29 a between P-body regions 26 a & 26 b down to N+ substrate 21.Similarly, drift region 22 b extends from gate oxide 29 b between P-bodyregions 26 c & 26 d down to N+ substrate 21.

Source electrode 32 is electrically connected to N+ source regions 27,which are disposed in respective P-body regions 26. For example, N+source region 27 a is disposed in P-body region 26 a; N+ region 27 b isdisposed in P-body region 27 b, and so on. It is appreciated that avariety of alternative source electrode connections are also possible.The area of the P-body regions directly beneath gate 30 (between N+source regions 27 and drift regions 22) comprises the IGFET channelregion of the transistor. In this particular embodiment, the gate regionis a metal-oxide semiconductor (MOS), and the IGFET is a NMOStransistor. Thus, the channel regions of HVFET 20 are defined at one endby N+ source regions 27 and at the other end by N-type drift regions 22,which extend vertically from gate oxide 29 down to the N+ substrate 21.Insulating layers 33 separate gate 30 from source electrode 32. Thedrift regions define a path for current flow, herein referred to as thefirst direction.

The n-type drift regions 22 are separated laterally by insulatingregions or dielectric layers 28. This direction of separation issubstantially orthogonal to the first direction and is herein referredto as the second direction. In the embodiment of FIG. 1, dielectriclayers 28 extend vertically from beneath P-body regions 26 down to N+substrate 21 along the full vertical length of the drift regions 22. Byway of example, dielectric layers 28 may comprise silicon dioxide, butother insulating materials, such as silicon nitride, may also be used.Disposed within each of the dielectric layers 28, and fully insulatedfrom the semiconductor substrate 21 and drift regions 22, is a fieldplate member 24. Field plate members 24 comprise a conducting layer ofmaterial such as heavily doped polysilicon, metal, metal alloys, etc. Asshown in the embodiment of FIG. 1, each of the field plate members 24 iselectrically connected to source electrode 32. Alternatively, the fieldplate members may be connected to a separate electrode. Gates 30 arealso connected to a separate electrode (not shown). Drain electrode 31provides electrical connection to the bottom of N+ substrate 21.

The extended drain region of vertical NMOS high-voltage transistor 20 ofFIG. 1 consists of a plurality of laterally interleaved layers of dopedsemiconductor material (e.g., n-type drift regions 22), insulatingmaterial (e.g., silicon dioxide dielectric layer 28), and conductingmaterial (e.g., heavily-doped polysilicon). In the on state, asufficient voltage is applied to the gate such that a channel ofelectrons is formed along the surface of the P-body regions 26. Thisprovides a path in the first direction for electron current flow fromsource electrode 32, N+ source regions 27, through the channel regionsformed in P-body regions 26, down through the N-type drift regions 22,through the N+ substrate 21, to drain electrode 31.

Practitioners in the semiconductor arts will note that in a conventionalvertical HVNMOS transistor, the N-type drift region is normally verythick (i.e., long) and lightly doped; both of which contribute to highon state resistance. In the device structure of FIG. 1, on the otherhand, the doping in the N-type drift regions may be considerably higher,such that the on-state resistance is dramatically lowered. Lowering theon-state resistance is achieved in HVFET 20 by the use of multiple,parallel-arranged extended drain or drift regions.

In the off state, a high voltage (e.g., 200V-1200V) is applied acrossthe respective drain and source electrodes 31 and 32. As the voltageincreases, the presence of field plate regions 24 on opposite sides ofdrift regions 22 cause the N-type drift regions to become depleted offree carriers. Ideally, the doping profile in the drift regions 22 istailored such that the resulting electric field is approximatelyconstant along the path from the drain to the source. For example, thedoping concentration may be highest near the N+ substrate 21, lowest thenear the P-body regions 26, and linearly graded in between.

The thickness of both the N-type drift regions 22 and oxide layers 28should be designed so as to guard against premature avalanche breakdown.Avalanche breakdown can be avoided by making the drift region relativelynarrow in the second direction, which reduces the ionization path andthereby increases the critical electric field at which avalanche occurs.In the same regard, making oxide layers 28 relatively wide in the seconddirection allows the device structure to support a larger voltage for agiven critical electric field.

By way of example, a device manufactured in accordance with FIG. 1having a drift region that is about 50 um high and about 0.4-0.8 umwide, with an oxide layer width in the approximate range of 3.0-4.0 umis capable of supporting about 800V. In such a device, the doping in thedrift region may be linearly graded from about 5×10¹⁵ cm⁻³ near theP-body regions to about 1×10¹⁷ cm⁻³ near the N+ substrate. The on-stateresistance of such a device is about 1.0 ohm-mm².

Practitioners in the art will appreciate that the device performance forHVFET 20 may be improved when manufactured as a smaller total cell pitch(i.e., combined width of field plate, oxide layer and drift regions)because the contribution of each drift region is fairly constant.

Referring now to FIG. 2, there is shown a lateral NMOS high-voltagetransistor 40 in accordance with another embodiment of the presentinvention. HVFET 40 of FIG. 2 operates according to the same principlesdiscussed in connection with the transistor structure of FIG. 1, exceptthat current flows laterally, as opposed to vertically, through thedrift regions. Note that in the embodiment of FIG. 2, field platemembers 44 are fully insulated from the semiconductor material by oxidelayers 49.

In this example, field plate member 44 a is disposed within oxide layer49 a just below the source and drain electrodes 46 and 45, respectively.Field plate member 44 b is disposed within oxide layer 49 b below N-typedrift region 42 a and above N-type drift region 42 b. The field platemembers may be connected to a field plate electrode at a certainlocation out of the plane of the figure. The N-type drift region, whichcomprises the extended drain of the transistor, extends laterally frombeneath P-body region 48 across to N+ drain region 43. N+ drain region43 connects both drift regions 42 a & 42 b with drain electrode 45.

An N+ source region 47, which is electrically connected to sourceelectrode 46, is disposed adjacent P-body region 48. The HVFET 40utilizes a vertical MOS gate structure 12 that comprises a gateelectrode 56 that connects to gate 55. In this embodiment, gate 55comprises a layer of polysilicon that extends vertically from gateelectrode 56. Gate 55 extends below the P-body region, and may extenddown to oxide layer 50, as shown. Gate 55 is insulated from N+ sourceregion 47, P-body region 48, and N-type drift region 42 by gate oxide53. An oxide region 58 separates gate electrode 56 from source electrode46.

Oxide layer 50 insulates N+ substrate 41 from gate 55, N-type driftregion 42, and N+ drain region 43. As can be seen, oxide layer 50extends laterally over N+ substrate 41 beneath each of the regions 42,43, and 55. Substrate electrode 57 provides electrical connection to thebottom of N+ substrate 41. The substrate may serve as the bottom fieldplate for drift region 42 b.

The on-state and off-state operations of HVFET 40 are similar to thosedescribed for the embodiment of FIG. 1. In this case, however, thesource and drain electrodes are located on the top surface. This meansthat electrons flows down through N+ source region 47, across thechannel region formed in P-body region 48 adjacent to gate oxide 53,laterally across the N-type drift regions 42, and up through the N+drain region 43 before reaching the drain electrode.

Note that even though FIG. 2 shows a trench gate structure, planar gatestructures could also be used. Additionally, a trench drain structurecould also be used in an alternative implementation. Furthermore,although the embodiment of FIG. 2 shows the extended drain regioncomprising two laterally-extending, parallel N-type drift regions 42 aand 42 b, other embodiments may utilize more than two parallel driftregions. In other words, the embodiment of FIG. 2 is not limited to justtwo drift regions, but could include any number of layers of drift,oxide, and field plate regions within manufacturing limits.

FIGS. 3A & 3B illustrate another embodiment of a lateral HVFET inaccordance with the present invention. FIG. 3A is a top view of alateral HVNMOS transistor 60, and FIG. 3B is a cross-sectional side viewof the same device, taken along cut lines A-A′, which extends throughdrift region 62 a. (Note that the source electrode 66, drain electrode65, gate 75, gate oxide 73 and oxide layer 79 are not depicted in FIG.3A to avoid confusion. These elements are shown in the cross-sectionalside view of FIG. 3B.)

The lateral device structure of FIG. 3 is similar to that shown in FIG.2. But rather than orient the drift, oxide, and field plate layeredregions on top of one another (vertically), the embodiment of FIG. 3 hasthese regions oriented side-by-side. Unlike the embodiment of FIG. 2,each of the N-type drift regions 62, oxide layers 69, and field platemembers 64 extend from underlying insulating layer 70 toward the uppersubstrate surface. Each of the N-type drift regions 62 and field platemembers 64 are insulated from N+ substrate 61 by insulating layer 70. Inone embodiment, layer 70 comprises silicon dioxide. An additionalelectrode 77 provides electrical connection to the bottom of N+substrate 61.

The planar gate and drain configurations of HVNMOS transistor 60 areillustrated in the side view of FIG. 3B. Alternatively, a trench drainstructure and/or a trench gate structure may be utilized. In thisembodiment, a gate member 75 is disposed above P-body region 68 and isinsulated from the semiconductor substrate by a gate oxide 73. Sourceelectrode 66 contacts N+ source region 67, which is disposed in P-bodyregion 68. P-body region 68 is itself shown disposed in N-type driftregion 62.

N+ drain region 63 is disposed at the opposite end of the N-type driftregion 62 and is electrically connected to drain electrode 65.

The embodiments of FIGS. 2 and 3 show the field plate members beingcoupled to the lowest chip potential, e.g., ground. The source may betied to the field plate members (at the lowest chip potential), or,alternatively, the source region may be left floating. In other words,the embodiments of FIGS. 1-3 are not limited to a source followerconfiguration. Each of the transistor structures of the presentinvention may be implemented as a four-terminal device, wherein thedrain, source, field plate members, and insulated gate members areconnected to a separate circuit terminal. In another embodiment, thefield plate and insulated gate members may be connected together.

With reference now to FIG. 4, there is shown a cross-sectional side viewof another embodiment of a vertical HVNMOS transistor 80 constructedaccording to the present invention. The device structure shown in FIG. 4is similar to that of FIG. 1, except that the planar gate has beenreplaced by a trench gate structure. As in the vertical device structureof FIG. 1, transistor 80 comprises a plurality of parallel-arrangedN-type drift regions 82 that extend vertically from P-body regions 86down to the N+ substrate 81. Each of the drift regions 82 is adjoined onboth sides by an oxide layer 88. For example, N-type drift region 82 ais bounded on one side by oxide layer 88 a and on the opposite side byoxide layer 88 b.

Disposed within each of the oxide layers 88, and fully insulated fromthe drift region and substrate semiconductor materials, is a field platemember 84 that may be electrically connected to source electrode 92. TheN-type drift regions 82, oxide layers 88, and field plate members 84collectively comprise a parallel layered structure that extends in alateral direction, which is perpendicular to the direction of currentflow in the on-state. When transistor 80 is in the on-state, currentflows vertically from the drain electrode 91 through the parallel N-typedrift regions 82, through the MOS channel formed on the sidewalls of theP-body region, to the source electrode 92.

The trench gate structure of vertical HVNMOS transistor 80 comprisesgate members 90 disposed between field plate members 84 and P-bodyregions 86. In the embodiment of FIG. 4, a pair of N+ source regions 87is disposed in each of P-body regions 86 on opposite sides. Each P-bodyregion 86 is located at one end of a corresponding N-type drift region82. A thin gate-insulating layer 89 (e.g., oxide) insulates each of gatemembers 90 (e.g., polysilicon) from the P-body semiconductor material.

For example, FIG. 4 shows gate members 90 a & 90 b disposed alongopposite sides of P-body region 86 a. N+ source regions 87 a & 87 b aredisposed in P-body region 86 a at opposite sides adjacent to the gatemembers; both regions 87 a & 87 b are electrically connected to sourceelectrode 92. P-body region 86 a adjoins the source electrode at one endand drift region 82 a at the other end. When transistor 80 is in theon-state conducting channel regions are formed along the sides of P-bodyregion 86 a such that current flows from source electrode 92, through N+regions 87, across P-body 86, down through N-type drift regions 82 andN+ substrate 81, to drain electrode 91.

Practitioners in the art will appreciate that the pair of N+ sourceregions 87 shown disposed in each P-body region 86 of FIG. 4 mayalternatively be replaced by a single N+ region that extends across thefull width of region 86 adjacent to source electrode 92. In this case,the P-body region may be connected to the source electrode at variouspoints (dimensionally into the page of the figure.) In one embodiment,source electrode 92 may protrude through N+ source 87 to contact theunderlying P-body region 86 (see FIG. 5K).

The trench gate structure of the embodiment of FIG. 4 potentially offersan advantage of a simplified manufacturing process, due to theelimination of the T-shaped semiconductor regions shown in FIG. 1. Also,the vertical HVNMOS structure of transistor 80 may provide loweron-resistance due to the elimination of the JFET structure formedbetween the P-body regions.

FIGS. 5A-5K illustrate the various processing steps that may be employedto fabricate a vertical high-voltage transistor in accordance with thepresent invention. The described fabrication method may be used not onlyto form the device of FIG. 5K, but also the vertical device structureshown in FIG. 4.

FIG. 5A shows a vertical high-voltage transistor after the initialprocessing step of forming an epitaxial layer 101 of n-typesemiconductor material on an N+ substrate 100. To support appliedvoltages in the range of 200V to 1000V the device structure should havean epitaxial layer that is about 15 um to 120 um thick. By way ofexample, the epitaxial layer of the device shown in FIG. 5 is 40 umthick. The N+ substrate 100 is heavily doped to minimize its resistanceto current flowing through to the drain electrode, which is located onthe bottom of the substrate in the completed device. Substrate 100 maybe thinned, for example, by grinding or etching, and metal may bedeposited on its bottom surface to further reduce the on-resistance ofthe transistor. Most often, these processing steps would be performedafter the topside processing has been completed.

The thickness and doping of epitaxial layer 101 largely determine theVbd of the device. The doping may be carried out as the epitaxial layeris being formed. The optimal doping profile is linearly graded from thedrain (at the bottom, adjacent to N+ substrate 100) to the source (atthe top). Tailoring the doping concentration so that it is heavier nearthe substrate 100 results in a more uniform electric-field distribution.Linear grading may stop at some point below the top surface of theepitaxial layer 101. By way of example, for the embodiment shown in FIG.5 the doping concentration is approximately 2×10¹⁵ cm⁻³ near the P-bodyregion to about 6×10¹⁶ cm⁻³ near the N+ substrate 100.

After the epitaxial layer 101 has been formed, the top surface of layer101 is appropriately masked and deep trenches are then etched into, oralternatively completely through, the epitaxial layer. FIG. 5B shows across-sectional view of the device structure following etching ofepitaxial layer 101 and part of substrate 100. Note that the lateralwidth of the etched trenches is determined by the combined thickness ofthe dielectric and conductive refill layers, as described below.

Spacing between adjacent trenches is determined by the requiredthickness of the remaining mesa of epitaxial layer material, which, inturn, is governed by the breakdown voltage requirements of the device.It is this mesa of epitaxial material that eventually forms the N-typedrift region of the device structure. It should be understood that thismesa of material might extend a considerable lateral distance in anorthogonal direction (into the page). Although the embodiment of FIG. 5illustrates a device having an extended drain region that comprises asingle N-type drift region, it is appreciated that the verticalhigh-voltage transistor of FIG. 5 may be constructed with a plurality ofparallel-arranged N-type drift regions. Ideally, it is desired to makethe lateral thickness (i.e., width) of the N-type drift region(s) asnarrow as can be reliably manufactured in order to achieve a very highVbd with a low Rsp. Of course, a larger lateral thickness is easier tomanufacture, but the specific on-resistance of the device suffers with alarger lateral thickness since the current is required to flow across alarger silicon area. In one implementation, the thickness is in theapproximate range of 0.4 to 1.2 microns. In this example, the thicknessof the mesa is about 1 um.

FIG. 5C shows the device structure of FIG. 5B after partial filling ofthe etched trenches with a dielectric material, e.g., silicon dioxide.As shown, in the embodiment of FIG. 5 oxide region 102 a covers one sideof etched epitaxial region 101, while oxide region 102 b covers theother side of epitaxial region 101. Oxide region 102 also covers the topsurface of N+ substrate 100 in each of the trenches.

The dielectric material may be introduced into the trenches using avariety of well-known methods. For instance, regions 102 may be grownthermally, deposited by chemical vapor deposition, and/or spun on inliquid form. For a given lateral thickness of epitaxial layer material101, the thickness of the dielectric layer may be set to provide arequired breakdown voltage, with thicker dielectric layers providing ahigher Vbd. However, thicker dielectric layers increase the cell pitchof the transistor structure and result in higher specific on-resistance.In one implementation, the 600V device structure of FIG. 5 has an oxidelayer lateral thickness of 4 um. For devices with other V_(bd)performance, this thickness may be in the range of about 2 um-5 um.

FIG. 5D illustrates the device structure of FIG. 5C following the stepsof filling the remaining portions of the trenches with a conductivematerial and planarizing the surface to form field plate regions 103.For example, the conductive material may comprise a heavily dopedpolysilicon, a metal (or metal alloys), and/or silicide. Conductorregions 103 a and 103 b form the field plate members of the device. Inmost cases, field plate members 103 a and 103 b should be made as narrowas can be reliably manufactured, since the field plate members occupysilicon area without directly contributing to device conductivity orbreakdown voltage characteristics. In one embodiment, the lateralthickness of field plate members 103 is approximately 0.5 um-1.0 um. Theplanarization of the surface may be performed by conventional techniquessuch as chemical-mechanical polishing.

At this point in the process, fabrication of the extended drain regionof the device is essentially complete. The remaining processing stepsmay be adapted to produce a stand-alone, high-voltage, depletion-modeMOSFET device structure (as shown in FIG. 5G and FIG. 6) or ahigh-voltage FET that incorporates a low-voltage MOSFET structure (e.g.,FIG. 5K), or other high-voltage devices.

FIG. 5E is a cross-sectional side view of the device structure of FIG.5D after the introduction of an N+ source region 105 at the top surfaceof epitaxial layer 101. Source region 105 may be formed using ordinarydeposition, diffusion, and/or implantation processing techniques.

After formation of the N+ source region 105 an interlevel dielectriclayer 106 is formed over the device. In the embodiment of FIG. 5,interlevel dielectric layer 106 comprises ordinary silicon dioxide thatmay be deposited and patterned by conventional methods. Openings areformed in dielectric layer 106 and a conductive layer of material (e.g.,metal, silicide, etc.) is deposited and patterned to produce thestructure shown in FIG. 5F. In this cross-sectional view, sourceelectrode 109 provides electrical connection to N+ source region 105,and electrodes 110 a and 110 b provide electrical connection to fieldplate members 103 a and 103 b, respectively.

FIG. 5G shows the device structure of FIG. 5F following formation of adrain electrode 111 on the bottom of N+ substrate 100. For example,drain electrode 111 may be formed using the conventional technique ofmetal sputtering. As described earlier, the bottom of the substrate mayfirst be subjected to grinding, implanting, etc., to lower the draincontact resistance.

The device of FIG. 5G represents a completed high-voltage transistorhaving a stand-alone drift region; that is, the device of FIG. 5G doesnot include a low-voltage, series MOSFET structure at the top of theepitaxial layer. Instead, the extended drift region formed by theepitaxial layer, itself, performs the function of the MOSFET without theinclusion of a P-body region. Practitioners in the arts will note thatin this device structure current cannot be completely turned-off, sincethere exists a continuous n-type path for electrons to flow from sourceelectrode 109 to drain electrode 111. Current flow in the devicestructure of FIG. 5G, however, does saturate when the mesa-likeepitaxial layer 101 is pinched-off at high drain voltages.

The device structure of FIG. 6 achieves pinch-off of the extended drainregion at lower voltages than the device of FIG. 5G. This is achieved byreducing the spacing between the field plate members 103 and epitaxiallayer 101 near the top of the N-type drift region, thereby increasingthe capacitance to pinch-off the vertical drift region at a relativelylow voltage. FIG. 6 shows a multi-tiered field plate structure extendinglaterally into oxide regions 102 a & 102 b to control the pinch-offvoltage and, therefore, the saturation current. Alternatively, the fieldplate members may comprise a single step, a linearly graded lateralextension, or some other profile shape designed to achieve the sameresult.

Those skilled in the arts will appreciated that for certain circuitapplications it may be advantageous to utilize the stand-alonetransistor structure of FIG. 5G (or FIG. 6) in series with an ordinaryexternal, low-voltage switching MOSFET. In such an application thelow-voltage (e.g., 40V) MOSFET could be used for switching purposes inorder to completely turn off current flow in the high-voltage (e.g.,700V) transistor device.

Referring now to FIGS. 5H-5K, there is shown an alternative processingsequence that may be used to fabricate a vertical HVNMOS transistor thatincludes an insulated gate MOS structure.

Trenches 112 a and 112 b are formed in respective dielectric layers 102a and 102 b on opposite sides of epitaxial layer 101 to accommodate theformation of the insulated gate structure. The depth of trenches 112 aand 112 b extends from the surface of N+ source region 105 to a depthgoverned by the intended MOSFET channel length and field platingconsiderations. In this example, the trench depth is about 1-5 um. Byway of example, trenches 112 may be formed by appropriate application ofa patterned masking layer to the semiconductor substrate followed byconventional dry or wet etching techniques into oxide layer 102.

FIG. 5J shows the device after formation of gate dielectric layers 116and gate members 113 within trenches 112. The gate dielectric layers 116a & 116 b may be formed by growing or depositing oxide on the sidewallsof the stacked N+ source, P-body, and epitaxial regions. The devicethreshold voltage determines the thickness of layers 116. In oneembodiment, layers 116 comprise silicon dioxide having a thickness onthe order of 250-1000 angstroms.

In the embodiment shown, a portion of dielectric layers 102 isolatesfield plate members 103 from gate members 113. Alternatively, trenches112 may expose the top portion of field plate 103 and the sameprocessing steps used to create layers 116 may also be used to formdielectric layers on the sidewalls of the field plates to isolate thefield plates from the gate members.

Once dielectric layers 116 have been formed on the sidewalls of trenches112, a conductive material, such as doped polysilicon, may be depositedto fill the remaining portions of the trenches. In this implementation,the doped polysilicon forms the gate members 113 a and 113 b of the MOStransistor structure. FIG. 5J shows the device after introduction of aP-body region 107 and a N+ source region 105 at the top surface ofepitaxial region 101. In the completed device, application of asufficient voltage to gate members 113 causes a conductive channel to beformed along the sidewall portions of P-body region 107 between N+source region 105 and epitaxial region 101. The channel length istherefore determined by the thickness of P-body region 107, which, forthe particular embodiment shown, may be approximately 0.5 um-3.0 um,with the N+ source region 105 in the range of about 0.1-0.5 um. Ashorter channel length results in a lower channel resistance, whichlikewise reduces the on-resistance of the device. It should beunderstood, however, that a too short channel would cause punch-throughproblems.

FIG. 5K shows the completed HVFET device structure following formationof an interlevel dielectric layer 106 (e.g., silicon dioxide, siliconnitride, etc.). This layer may be deposited and patterned to formcontact openings. In the embodiment shown, the etching of layer 106 isfollowed by etching of the field plates, gate members, N+ and P-bodyregions. This is followed by deposition and patterning of a conductivelayer (e.g., metal, silicide, etc.) to create source electrode 109, gateelectrodes 115, and field plate electrodes 110, which provide electricalconnection to the respective regions of the device. The optional etchingstep described above allows the source electrode to contact the P-bodyregion without patterning the N+ source region, thus simplifying theprocess. A conductive layer may also be applied to the bottom ofsubstrate 100 (after optional treatment by grinding, etching,implanting, etc.) to form the drain electrode 111.

Note that while source electrode 109 is shown extending down to P-body107 in the cross-sectional view of FIG. 5K, in other embodimentselectrode may only extend to the upper surface of source region 105. Itshould also be appreciated that electrode 109 does not separate region105 into two separate source regions in the illustration of FIG. 5K.Rather, electrode 109 is fabricated in the form of a plug that issurrounded by N+ material that comprises region 105.

1. A method comprising: forming, in a semiconductor substrate of a firstconductivity type, first and second trenches that define a mesa havingrespective first and second sidewalls, the mesa comprising an epitaxiallayer formed with a doping concentration that is lower near an upperportion, as compared to a lower portion, of the epitaxial layer;partially filling each of the trenches with a dielectric material thatcovers the first and second sidewalls; filling a remaining portion ofthe trenches with a conductive material to form first and second fieldplates in the first and second trenches, respectively, the first andsecond field plates extending vertically from near a bottom of the mesato a top surface of the semiconductor substrate; forming source and bodyregions in an upper portion of the mesa, the source region being of thefirst conductivity type and the body region being of a secondconductivity type opposite to the first conductivity type, the bodyregion separating the source from a lower portion of the mesa, the lowerportion of the mesa comprising a drift region, the dielectric materialand the field plates being formed with a reduced spacing between thefield plates and the mesa near the body region as compared to near thelower portion of the mesa; and forming a gate embedded within thedielectric material adjacent the body region, the gate being insulatedfrom the body region and the first and second field plates.
 2. Themethod of claim 1 further comprising: forming a drain region of thefirst conductivity type at the bottom of the mesa, the drain regionbeing connected to the drift region; forming a source electrodeconnected to the source region; and forming a drain electrode connectedto the substrate.
 3. The method of claim 1 wherein the firstconductivity type is n-type.
 4. The method of claim 1 wherein the driftregion has a linearly graded doping profile.
 5. A method comprising:forming, in a semiconductor substrate of a first conductivity type,first and second trenches that define a mesa having respective first andsecond sidewalls, the mesa comprising an epitaxial layer formed with alinearly graded doping profile; partially filling each of the trencheswith a dielectric material that covers the first and second sidewalls;filling a remaining portion of the trenches with a conductive materialto form first and second field plates in the first and second trenches,respectively, the first and second field plates extending verticallyfrom near a bottom of the mesa to a top surface of the semiconductorsubstrate; forming source and body regions in an upper portion of themesa, the source region being of the first conductivity type and thebody region being of a second conductivity type opposite to the firstconductivity type, the body region separating the source from a lowerportion of the mesa, the lower portion of the mesa comprising a driftregion, the dielectric material and the field plates being formed with areduced spacing between the field plates and the mesa near the bodyregion as compared to near the lower portion of the mesa; and forming agate embedded within the dielectric material adjacent the body region,the gate being insulated from the body region and the first and secondfield plates.
 6. The method of claim 5 further comprising: forming adrain region of the first conductivity type at the bottom of the mesa,the drain region being connected to the drift region; forming a sourceelectrode connected to the source region; and forming a drain electrodeconnected to the substrate.
 7. The method of claim 5 wherein the firstconductivity type is n-type.
 8. The method of claim 5 wherein thelinearly graded doping profile of the epitaxial layer is formed suchthat the mesa has a doping concentration that is lower near the bodyregion, as compared to a lower portion of the epitaxial layer.